Ancient China sheds 'light' on a red star

ByRobert C. CowenDecember 30, 1981

At this season in the Northern Hemisphere, Orion the Hunter dominates the night sky. The famous star Betelgeuse gleams like a rare orange sapphire on his right shoulder.

Astronomers now wonder if he may have worn other jewels within the astronomically brief period encompassed by historical records.

Classed as a red giant, Betelgeuse (pronounced ''beetle-juice'') actually is more orange than red in color. But there is no way you could call it yellow, let alone white. Yet Chinese records from the first century BC refer to the star as being white or yellow.

Commenting on this in Chinese Astronomy and Astrophysics, a relatively new English language journal, Chinese astrophysicist Fang Li-zhi suggests that the star has only recently become a red giant. If so, this would give astrophysicists a rare opportunity to study a star that has just embarked on one of the most interesting phases of its evolution.

As stars use up the hydrogen fuel in their cores, they begin to expand. Their average density falls as their matter is spread over an increasing volume. Their luminosity rises, while their surface temperature drops and their color changes. Typically, they go from white, through yellow, to red.

There seems little doubt that Betelgeuse now is in the red giant phase. With a mass of anywhere from five to 20 times that of the sun, it is large enough to fill at least the orbit of Mars and perhaps that of Jupiter. Its average density is ten-thousand times rarer than Earth's air at sea level, making Betelgeuse what some astronomers call a ''red-hot vacuum.''

The historical references were published in China a few years ago by Bo Shu-ren, Wang Jian-min, and Liu Jin-yi. Two of them sent some of these references to astrophysicist Kenneth Brecher of Boston University. Brecher, noting that Ptolemy called Betelgeuse red in AD 150, suggested two years ago that the star's color may have been temporarily changed when it expelled a shell of dust and gas that, even now, can be seen to be expanding away from it. However, Fang Li-zhi, in his more recent discussion, suggests that the star has actually undergone a rapid evolution with its true color changing from white or yellow to orange-red.

References in the Chinese records to Sirius, Orion's dog, as appearing white as it does today are also intriguing. A number of classical Western writers, including Ptolemy, refer to Sirius as being ''red'' or ''ruddy.'' This has aroused speculation that a companion star to Sirius may have been a red giant that rapidly degenerated into the small white dwarf known today. Since this contradicts theories of stellar evolution, astronomers would rather think that the classical writers simply were wrong - a conclusion which the Chinese records tend to support.

In spite of the sophisticated means we have for studying the cosmos, it seems there still is something we can learn from our ancestors.